codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>form va e vieni</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<script type="text/javascript">
function Attiva(sel){
if(sel.options[sel.selectedIndex].value=='sett 1' || sel.options[sel.selectedIndex].value=='sett 2'){
document.getElementById('sel1').style.display='block';
document.getElementById('sel2').style.display='none';
}else if(sel.options[sel.selectedIndex].value=='sett 3' || sel.options[sel.selectedIndex].value=='sett 4'){
document.getElementById('sel2').style.display='block';
document.getElementById('sel1').style.display='none';
}else{
document.getElementById('sel1').style.display='none';
document.getElementById('sel2').style.display='none';
}
}
</script>
<style type="text/css">
#sel1, #sel2{
	display: none;
}
</style>
</head>

<body bgcolor="#8692A5">
<h2>Il form di prova</h2>
<form name="modulo" method="post" action="">

<table width="100%" border="0" cellpadding="2" cellspacing="0" bgcolor="#8692A5">
<tr valign="top">
 <td width="31%">Nome</td>
 <td width="69%"><input type="text" name="FieldData0" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Cognome</td>
 <td><input type="text" name="FieldData1" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Citt&agrave;</td>
 <td><input type="text" name="FieldData2" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Settori</td>
 <td>
<select name="trasfer" id="trasfert" onchange="Attiva(this)">
<option value="">==</option>
<option value="sett 1">Settore 1</option>
<option value="sett 2">settore 2</option>
<option value="varie 1">Varie 1</option>

<option value="varie 2">Varie 2</option>
<option value="sett 3">Settore 3</option>
<option value="sett 4">Settore 4</option>
<option value="varie 3">Varie 3</option>
<option value="varie 4">Varie 4</option>
</select>
qui le scelte da 

effettuare per mostrare o
 
nascondere i campi aggiuntivi </td>
 </tr>

<tr valign="top">
 <td>Nazione</td>
 <td><input type="text" name="FieldData3" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Azienda</td>

 <td><input type="text" name="FieldData4" size="30"> </td>
 </tr>

<tr valign="top">
 <td colspan="2">
	<table width="100%" id="sel1">
	<tr>
	<td width="31%">nome</td>

	<td width="69%"><input type="text" name="textfield" id="textfield"></td>
	</tr>
	<tr>
	<td>cognome</td>
	<td><input type="text" name="textfield2" id="textfield2"></td>
	</tr>
	<tr>
	<td>città</td>

	<td><input type="text" name="textfield3" id="textfield3"></td>
	</tr>
	</table>
	<table width="100%" id="sel2">
	<tr>
	<td width="31%">nome e cognome</td>

	<td width="69%"><input type="text" name="textfield4" id="textfield4"></td>
	</tr>
	<tr>
	<td>città</td>
	<td><input type="text" name="textfield5" id="textfield5"></td>
	</tr>
	<tr>
	<td>indirizzo</td>

	<td><input type="text" name="textfield6" id="textfield6"></td>
	</tr>
	<tr>
	<td>CAP</td>
	<td><input type="text" name="textfield7" id="textfield7"></td>
	</tr>
	<tr>
	<td>provincia</td>

	<td><input type="text" name="textfield8" id="textfield8"></td>
	</tr>
	</table>
    </td>
 </tr>

<tr valign="top">
 <td>Indirizzo eMail</td>
 <td><input type="text" name="FieldData5" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Telefono</td>
 <td><input type="text" name="FieldData6" size="30"> </td>
 </tr>

<tr valign="top">
 <td>Fax</td>
 <td><input type="text" name="FieldData7" size="30"> </td>
 </tr>
<tr>
 <td colspan="2"></td>
 </tr>
<tr>
<td align="right" colspan=2 >
<input type="text" name="hida2" value="" maxlength="100" size="3">
<input type="submit" class="btn" value="Invia" name="Submit">  <input type="reset" class="btn" value="  Azzera  " name="Clear"></td>
</tr>
<tr>
<td colspan=2 align="center"></td></tr>
</table>
</form>


</p>


</p>
</body>
</html>